Method and device for monitoring a sensor

ABSTRACT

A device and a method for monitoring a sensor for an exhaust-gas aftertreatment system, in particular a temperature sensor, are described. In specified operating states a first signal from the first sensor to be monitored is compared to a second signal from a second sensor. Errors are detected when at least the two signals differ from one another by more than a value.

BACKGROUND INFORMATION

[0001] The present invention relates to a method and a device for monitoring a sensor.

[0002] A method and a device for controlling an internal combustion engine having an exhaust-gas aftertreatment system is known from German Patent 199 06 287. In the system described therein, a particle filter is used which filters out particles contained in the exhaust gas. In order to accurately control the internal combustion engine and the exhaust-gas aftertreatment system, the state of the exhaust-gas aftertreatment system must be known. Sensors, among other devices, are used to determine the state of the exhaust-gas aftertreatment system. Sensors are used in particular to provide the temperature values which characterize the temperature upstream from, downstream from, and/or in the exhaust-gas aftertreatment system.

ADVANTAGES OF THE INVENTION

[0003] A method of monitoring a sensor for an exhaust-gas aftertreatment system, in particular a temperature sensor. It is possible to easily and accurately monitor sensors for an exhaust-gas aftertreatment system by comparing, in specified operating states, a first signal from a first sensor to be monitored to a second signal from a second sensor, and detecting errors when at least the two signals differ from one another by more than a predeterminable value. Temperature sensors are preferably monitored using the method. However, the method may also be used for other sensors which are used for controlling the exhaust-gas aftertreatment system. This preferably applies to sensors for detecting the state of the exhaust-gas aftertreatment system, and/or for pressure, temperature, and/or air flow sensors.

[0004] The use of the method for temperature sensors is particularly advantageous. In particular, the first signal characterizes the temperature of the exhaust gases upstream from an exhaust-gas aftertreatment system, in an exhaust-gas aftertreatment system, and/or downstream from an exhaust-gas aftertreatment system. The detection of these temperature variables is particularly sensitive, since controlling the exhaust-gas aftertreatment system requires that these variables be detected with high accuracy.

[0005] It is particularly advantageous when these signals are compared to a second signal which characterizes the temperature of the gases fed to the internal combustion engine, to the oxidation catalyst, or to the particle filter. These signals are suitable in particular, since in specified operating states this comparison signal takes on essentially the same values as the signals to be monitored.

[0006] It is particularly advantageous to monitor in operating states for which the cooling water temperature and/or the temperature of the gases fed to the internal combustion engine and/or the difference between the two temperatures is less than a threshold value. In these operating states, the difference between the signals to be monitored and the comparison signals is at a minimum.

[0007] Other particularly advantageous embodiments are characterized in the subclaims.

DRAWING

[0008] The present invention is explained below, with reference to the embodiment illustrated in the drawing.

[0009]FIG. 1 shows a block diagram of the device according to the present invention; and

[0010]FIG. 2 shows a flow chart for clarifying the method according to the present invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0011] The device according to the present invention is illustrated below using an auto-ignition internal combustion engine as an example, in which the fuel metering is controlled by a common rail system. However, the method according to the invention is not limited to these systems, and may be used in other internal combustion engines as well.

[0012] An internal combustion engine designated by reference number 100 receives fresh air supplied via an intake line 102 and discharges exhaust gases via an exhaust line 104. Exhaust-gas aftertreatment means 110 are situated in exhaust line 104 from where the purified exhaust gases are released to the atmosphere via line 106. Exhaust-gas aftertreatment means 110 essentially include a precatalyst 112 and a filter 114 downstream therefrom. A temperature sensor 124 is preferably situated between precatalyst 112 and filter 114 which provides a temperature signal TVF. Sensors 120 a and 120 b are respectively provided upstream from precatalyst 112 and downstream from filter 114. These sensors act as differential pressure sensors 120 and provide a differential pressure signal DP which characterizes the differential pressure between the inlet and the outlet of the exhaust-gas aftertreatment means. In addition, a sensor 126 is situated in intake line 102 which detects a signal T1 which characterizes temperature T1 of the fresh air supplied. A sensor 125 transmits a signal TVO which characterizes the temperature upstream from exhaust-gas aftertreatment system 110.

[0013] Fuel is metered to internal combustion engine 100 via a fuel metering device 140. The fuel metering device meters fuel to the individual cylinders of internal combustion engine 100 via injectors 141, 142, 143, and 144. The fuel metering device is preferably a common rail system. A high-pressure pump conveys fuel to an accumulator. From the accumulator the fuel enters the internal combustion engine via the injectors.

[0014] Various sensors 151 are situated on fuel metering device 140 which provide signals that characterize the state of the fuel metering device. For a common rail system, for example, this involves pressure P in the accumulator. Sensors 152 are situated on internal combustion engine 100 which characterize the state of the internal combustion engine. This involves, for example, a temperature sensor which provides a signal TW that characterizes the engine temperature.

[0015] The output signals from these sensors reach a control 130, which is illustrated as a first partial control 132 and a second partial control 134. Preferably, the two partial controls form a structural unit. First partial control 132 preferably actuates fuel metering device 140 with actuating signals AD which influence the fuel metering. To this end, first partial control 132 contains a fuel metering control 136. The fuel metering control transmits a signal ME, which characterizes the quantity to be injected, to second partial control 134.

[0016] Second partial control 134 preferably controls the exhaust gasaftertreatment system, and to this end detects the corresponding sensor signals. In addition, second partial control 134 exchanges signals, in particular regarding quantity of fuel ME injected, with first partial control 132. The two controls preferably make mutual use of the sensor signals and the internal signals.

[0017] The first partial control, also referred to as engine control 132, controls actuating signal AD for actuating fuel metering device 140 as a function of various signals which characterize the operating state of internal combustion engine 100, the state of fuel metering device 140, and the ambient conditions, in addition to a signal which characterizes the desired power and/or torque from the internal combustion engine. Such devices are known, and are used in a variety of ways.

[0018] Particle emissions may appear in the exhaust gas, in particular with diesel engines. To this end, exhaust-gas aftertreatment means 110 filter these particles from the exhaust gas. Particles accumulate in filter 114 as a result of this filtering process. To clean the filter, these particles are then combusted in specified operating states, load states, and/or after specified times or counter positions have passed for the quantity of fuel or the distance traveled. Filter 114 is usually regenerated by increasing the temperature in exhaust-gas aftertreatment means 110 until the particles combust.

[0019] Precatalyst 112 is provided for raising the temperature. The temperature is raised by, for example, increasing the proportion of uncombusted hydrocarbons in the exhaust gas. These uncombusted hydrocarbons then react in precatalyst 112 and raise the temperature of the precatalyst and that of the exhaust gas.

[0020] This temperature increase in the precatalyst and the exhaust gas requires an increase in fuel consumption, and therefore should be carried out only when necessary, i.e., when filter 114 is loaded with a certain amount of particles. One possibility for determining the load state is to determine differential pressure DP between the input and the output of the exhaust-gas aftertreatment means, and/or to make a calculation and, based on this value, to determine the load state.

[0021] The demands on the efficiency and the availability of the individual components are very high. The sensors used, in particular the temperature sensors which are indispensable for proper control of the exhaust-gas aftertreatment system, play a key role in controlling the exhaust-gas aftertreatment system. It is not sufficient to test the sensors as part of maintenance or during a technical check for sensor operability. This is particularly relevant with regard to regulatory requirements for on-board diagnosis of emissions-related systems.

[0022] A defect in a sensor must be detected early to prevent exceedance of the allowable emissions and to ensure the operability of the exhaust-gas aftertreatment system. The method described below allows a simple check, in particular of the temperature sensors in the exhaust gas train, thereby ensuring the operability of the exhaust-gas aftertreatment system. In this manner, a failure and/or unacceptable drift of the sensors may be detected early.

[0023] The method according to the present invention is described below, using temperature sensors as an example. In principle, this method may also be used for other sensors, in particular for sensors in the exhaust gas train. The method according to the present invention may be used to check all sensors downstream from the internal combustion engine, i.e., between the internal combustion engine and the exhaust-gas aftertreatment system, sensors in the exhaust gas treatment, i.e., between the oxidation catalyst and the particle filter, and/or sensors downstream from the particle filter. The method according to the present invention may also be used for other systems of catalysts and/or particle filters.

[0024] According to the present invention, malfunctioning of the temperature sensor in the exhaust gas train is detected by determining the plausibility of individual signals when a cold engine is started. In this case it is assumed that all temperature signals are within the range of the ambient temperature. All necessary signals and data are already present in the control unit, so that no additional sensors are required.

[0025]FIG. 2 illustrates as a flow chart one embodiment of the method according to the present invention. A first query 200 checks whether sufficient time has elapsed since the last start. A second query 205 checks whether the internal combustion engine has begun to turn. Preferably, query 205 checks whether speed N is greater than 0. If this is not the case, step 205 is repeated. If the internal combustion engine is turning, a time counter Z is started in step 208, and a query 210 checks whether various temperature signals, such as cooling water temperature TW, and/or temperature T1 of fresh air drawn in, and/or the ambient temperature which is an approximation of temperature T1, are less than a threshold value SW. If this is not the case, i.e., if one of the temperatures is greater than the threshold value, it is determined in step 295 that a check is not possible. If the temperatures are less than the threshold value, query 220 follows. In one simplified embodiment, it may also be provided that only one of the temperature values is checked.

[0026] Query 220 checks whether the absolute value of the difference between these two temperature values is less than a threshold value SW2. It this is not the case, i.e., if the cooling water temperature and the ambient air temperature differ from one another significantly, it is likewise determined that no check is possible. If the two temperature values are approximately equal, query 230 follows.

[0027] Query 230 checks whether speed N is greater than a threshold value SN. If this is not the case, step 230 is repeated. If query 230 determines that the starting cutout speed has been reached, query 240 follows. This query 240 checks whether time Z elapsed since the first turning of the internal combustion engine is less than a threshold value ST. If this is not the case, it is likewise determined that no check is possible. If the time elapsed since the first turning of the internal combustion engine is less than a threshold value, step 250 is followed by the actual checking of the temperature sensors.

[0028] A first query 250 checks whether the absolute value of the difference between fresh air temperature T1 and temperature TVO upstream from the exhaust-gas aftertreatment system is greater than a threshold value GW1. If this is the case, errors are likewise detected in step 280. If this is not the case, a check is made as to whether the absolute value of the difference between ambient temperature T1 and temperature TVF in the exhaust-gas aftertreatment system is greater than a threshold value GW2. If this is the case, errors are likewise detected in step 280. If this is not the case, query 270 checks whether the absolute value of the difference between temperature signal TVO for the temperature upstream from the exhaust-gas aftertreatment system and temperature TVF in the exhaust-gas aftertreatment system is greater than a threshold value GW3. If this is the case, an error is likewise detected in step 280. If this is not the case, it is determined in step 290 that no error is present and the sensors are operating properly.

[0029] The described method illustrates the monitoring using the example of a temperature sensor 125 upstream from the exhaust-gas aftertreatment system and a temperature sensor 124 in the exhaust-gas aftertreatment system which is situated between an oxidation catalyst and a particle filter. The described method is not limited to this specialized system of sensors. The method may also be used for other systems of sensors and/or catalysts and filters. In particular for the use of an NO_(x) storage catalyst, it is possible to replace a temperature sensor, in particular temperature sensor 125, by a sensor upstream from the NO_(x) storage catalyst.

[0030] Furthermore, additional sensors may be situated in the exhaust-gas aftertreatment system, these sensors then being checked against the ambient temperature as well as with respect to one another for plausibility. In addition, in one simplified embodiment, it is also possible to provide only one sensor in the exhaust-gas aftertreatment system, and this sensor is checked against the ambient air temperature for plausibility.

[0031] Preferably, a check is made only with a cold start. That is, the check is made only if the engine has not been operated for a fairly long time. To this end, a check is made as to whether sufficient time has elapsed since the last start-up. This is determined by query 200. In this case a check is prevented, since the temperatures in the exhaust gas system have already been modified to the extent that an error was erroneously determined.

[0032] In addition, a check is made as to whether cooling water temperature TW and/or intake air temperature T1 and/or their difference is below a limit value SW. In addition, plausibility checking is not performed until starting cutout speed SN is exceeded and the internal combustion engine is in normal operation. This is ensured by query 230.

[0033] At this point the measured temperatures have been changed only insignificantly, which is attributed to sensor lag, among other factors. If the start-up takes too long, it is no longer possible to make this assurance. Therefore, a check is made in query 240 as to whether the starting cutout speed has been reached within a predeterminable maximum time.

[0034] Plausibility checking is preferably performed only when all checks have been carried out. For simplified embodiments, it may also be provided that one or the other query is not performed. The temperature differences between the temperature sensors to be monitored and the intake air temperature, and the difference between the temperature signals to be individually monitored, constitute the above-referenced prerequisites. In the embodiment illustrated, the temperature sensors upstream from the catalyst and upstream from the filter are monitored. If the absolute values of the differences are greater than a limiting value, an error is detected. An error-free state is determined when all temperature sensors indicate the value of a reference sensor. Instead of the intake air temperature, the ambient air temperature may be used as the reference sensor. 

What is claimed is:
 1. A method of monitoring a sensor for an exhaust-gas aftertreatment system, in particular a temperature sensor, a first signal from the first sensor to be monitored being compared to a second signal from a second sensor in specified operating states, errors being detected when at least the two signals differ from one another by more than a value.
 2. The method as recited in claim 1, wherein the first signal characterizes the temperature of the exhaust gases upstream from an exhaust-gas aftertreatment system, in an exhaust-gas aftertreatment system, and/or downstream from an exhaust-gas aftertreatment system.
 3. The method as recited in claim 2, wherein the first signal characterizes the temperature of the exhaust gases upstream from a catalyst, in particular an oxidation catalyst, or upstream from a particle filter.
 4. The method as recited in one of the preceding claims, wherein the second signal characterizes the temperature of the gases fed to the internal combustion engine, to the catalyst, in particular the oxidation catalyst, or to the particle filter.
 5. The method as recited in one of the preceding claims, wherein the specified operating states are present when the cooling water temperature and/or the temperature of the gases fed to the internal combustion engine, and/or the difference between the two temperatures, is less than a threshold value.
 6. The method as recited in one of the preceding claims, wherein the specified operating states are present when the speed is greater than a starting cutout speed.
 7. The method as recited in one of the preceding claims, wherein the specified operating states are present when the time elapsed since the last start-up is greater than a threshold value.
 8. The method as recited in one of the preceding claims, wherein the specified operating states are present when the duration of the start-up is less than a threshold value.
 9. A device for monitoring a sensor for an exhaust-gas aftertreatment system, in particular a temperature sensor, comprising means which in specified operating states compare a first signal from the first sensor to be monitored to a second signal from a second sensor, and which detect errors when at least the two signals differ from one another by more than a value. 